Sensor geometry for improved package stress isolation

ABSTRACT

The sensor geometry for improved package stress isolation is disclosed. A counterbore on the backing plate improves stress isolation properties of the sensor. The counterbore thins the wall of the backing plate maintaining the contact area with the package. The depth and diameter of the counterbore can be adjusted to find geometry for allowing the backing plate to absorb more package stresses. Thinning the wall of the backing plate make it less rigid and allows the backing plate to absorb more of the stresses produced at the interface with the package. The counterbore also keeps a large surface area at the bottom of the backing plate creating a strong bond with the package.

This application is a continuation of co-pending U.S. patent applicationSer. No. 11/973,966, filed Oct. 11, 2007, and entitled “SENSOR GEOMETRYFOR IMPROVED PACKAGE STRESS ISOLATION”, which is incorporated herein byreference.

TECHNICAL FIELD

Embodiments are generally related to sensing systems and methods.Embodiments are also related to pressure sensing systems such aslow-pressure medical sensors. Embodiments are additionally related tosensor geometry for improved package stress isolation.

BACKGROUND OF THE INVENTION

Micro-Electrical-Mechanical-Systems (MEMS) such as sensors can be widelyused in applications such as automotive, household appliance, buildingventilation, and in general industrial applications to sense a physicalcondition such as pressure, temperature, or acceleration, and to providean electrical signal representative of the sensed physical condition.Conventional pressure sensor is constructed as a network of resistors ina resistive bridge configuration, wherein the resistive bridge has twoterminals for coupling to power supply potentials and two terminals forproviding a differential output signal.

A drawback of resistive bridge type sensor is that they produce anon-zero output electrical signal (i.e., offset voltage) at their outputterminals with a null input applied. Temperature Coefficient of Offset(TCO) is a measure of non-pressure induced stresses as a function oftemperature that is placed on a semiconductor device such as MEMS deviceand is expressed in microvolts per degree Celsius.

In one prior art a non-zero TCO in a semiconductor is adjusted byreducing the amount of adhesive material utilized to secure a firststructure to a second structure. An adhesive layer utilized to secure asensor die to a constraint die in a pressure sensor application isreduced in thickness and/or formed so that adhesive material does notcompletely cover the constraint die. The TCO is further adjusted byreducing the amount and/or patterning the adhesive layer employed tosecure the sensor to its package.

In another prior art, a structure and method of making a piezoresistivetransducer with reduced offset current are disclosed. The transducer iscomprised of a piezoresistive die having a support rim and a diaphragm,and a support housing having a wall and an aperture. The shape of thediaphragm is matched with the shape of the aperture while the shape ofthe supporting rim is matched with the shape of the wall. By matchingthese shapes, temperature induced stresses are reduced, thus reducingtemperature induced offset currents.

Another prior art includes a package having a stress sensitivemicrochip, package modulus of elasticity, and an isolator between themicrochip and the package. The isolator has an isolator modulus ofelasticity that has a relationship with the package modulus ofelasticity. This relationship causes no more than a negligible thermalstress to be transmitted to the microchip.

Referring to FIG. 1, when a pressure die 102 is attached to a package101, the Coefficient of Thermal Expansion (CTE) mismatch between thedifferent materials produce package stresses that lead to an offsetsignal on the pressure die 102. Most of these stresses are produced atthe interface between the die 102 and another package 105. TemperatureCoefficient of Offset (TCO) can be a non-pressure induced signal as afunction of temperature on a pressure sensor. To minimize the TCO abacking plate 103 with a CTE close to silicon can be placed between thesilicon die 102 and the package. The backing plate 103 isolates thesensing die 104 from the package stresses. The Room TemperatureVulcanizing (RTV) or other die attached material for example having CTEof 4.4e-5 1/C can be used. Backing plate 103 can be made of Pyrex,Borofloat and Silicon or other material with a CTE close to silicon CTE.The packages 101 and 105 for example can have CTE of 1.6e-5 1/C and1.4e-5 1/C respectively.

Usually the backing plate's thickness is increased to minimize thepackage stresses on the silicon die. Package size, assembly processesand cost limitations limit the thickening of the backing plate. It iswell known in the industry that increasing the backing plate's thicknessimproves its stress isolation properties. However due to spacelimitation and electrical connection processes it is often not possiblefor the thickness to be increased.

Based on the foregoing, it is believed that a need exists to modify thebacking plate's geometry for improving the stress isolation withoutincreasing the thickness.

BRIEF SUMMARY

The following summary is provided to facilitate an understanding of someof the innovative features unique to the embodiments disclosed and isnot intended to be a full description. A full appreciation of thevarious aspects of the embodiments can be gained by taking the entirespecification, claims, drawings, and abstract as a whole.

It is, therefore, one aspect of the present invention to provide for animproved pressure sensor usable, for example, as low pressure medicalsensors.

It is another aspect of the present invention to provide for a sensorgeometry for improved package stress isolation.

The present invention utilizes a counterbore on the backing plate toimprove its stress isolation. This reduces the backing plate's stiffnessand allows it to absorb the stresses from the package. The depth anddiameter of the counterbore can be adjusted to provide a geometry thatallows a back plate to absorb more package stresses. The counterborethins the wall of the backing plate while maintaining the contact areawith the package. Thinning the wall of the backing plate make it lessrigid and allows the backing plate to absorb more of the stressesproduced at the interface with the package. The counterbore also keeps alarge surface area at the bottom of the backing plate where it attachesto the package. The larger surface area enables a strong bond with thepackage.

A counterbore makes it possible in a sensor package to employ thinnerbacking plates and thereby minimize package stresses on a silicon die.Package space constraints limit the total thickness of thesilicon/backing plate. In addition the ability to make robust electricalconnections to the die such as wirebonds to a substrate can be made farmore difficult with a taller die. The counterbore makes it possible toutilize a thinner backing plate that performs better than a thickerbacking plate.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, in which like reference numerals refer toidentical or functionally-similar elements throughout the separate viewsand which are incorporated in and form a part of the specification,further illustrate the embodiments and, together with the detaileddescription, serve to explain the embodiments disclosed herein.

FIG. 1, labeled “prior art”, illustrates a prior art representation ofperspective view of a conventional pressure sensor depicting a pressuredie on a package;

FIG. 2 illustrates a perspective view of a pressure sensor depictingcounterbore on the backing plate to improve backing plate stressisolation on a sense die, which can be implemented in accordance with apreferred embodiment;

FIG. 3 illustrates a perspective view of a pressure sensor depictingbending of backing plate and absorbing of the stress produced at thedie/package interface, which can be implemented in accordance with apreferred embodiment;

FIG. 4 illustrates a perspective view of a pressure sensor depictingcounterbore depth, which can be implemented in accordance with apreferred embodiment;

FIG. 5 illustrates a graph depicting variation of TCO with respect tocounterbore depth, which can be implemented in accordance with apreferred embodiment; and

FIG. 6 illustrates a high level flow chart depicting the process ofreducing TCO utilizing counterbore, in accordance with an alternateembodiment.

DETAILED DESCRIPTION

The particular values and configurations discussed in these non-limitingexamples can be varied and are cited merely to illustrate at least oneembodiment and are not intended to limit the scope thereof.

FIG. 2 illustrates a perspective view of a pressure sensor 200 depictingcounterbore 202 on the backing plate 205 to improve backing plate stressisolation, which can be implemented in accordance with a preferredembodiment. Most of the thermal stresses on the die 206 are produced atthe interface 203 with the package 204. A backing plate 205 with a CTEclose to silicon is typically placed between the package 204 and the die206 to isolate the die 206 from the package stresses.

The backing plate 205 is typically made of materials such as Silicon,Pyrex Hoya and Borofloat. A well-known technique to improve the backingplate's stress isolation is to make it thicker. A thicker backing plate205 puts the piezoresistors 201 farther away from the source of most ofthe packaging stresses. FIG. 2 also shows that the package 204 limitsthe thickness of the backing plate 205. The counterbore 202 techniqueprovides another way to improve the backing plate's 205 stressisolation. Without increasing the backing plate's thickness the stressisolation is improved with a counterbore 202 on the backing plate 205.The counterbore 202 decreases the backing plate's stiffness, whichtransmits less stress to the silicon die 206.

FIG. 3 illustrates a perspective view of a pressure sensor 200 depictingbending of backing plate 205 and absorbing of the stress produced at thedie/package (not shown) interface 203 depicted in FIG. 2, which can beimplemented in accordance with a preferred embodiment. FIG. 3 shows thestresses on the pressure die produced when the sensor is heated fromroom temperature to 185 degree Celsius. Note that in FIG. 2 and FIG. 3,identical or similar parts or elements are indicated by identicalreference numerals. Thus, FIG. 3 also contains the counterbore 202 andbacking plate 205. The counterbore 202 decreases the stiffness of thebacking plate 205. A less rigid backing plate 205 bends and absorbs thestress produced at the die/package (not shown) interface 203.Counterbore 202 isolate the silicon die 206 depicted in FIG. 2 from thepackage 204 depicted in FIG. 2 without making the backing plate thicker.This provides another way to modify the backing plate 205 so it can fitinside of the package 204. The cost of the backing plate 205 can also bereduced as the thickness increases cost. Counterbore 202 also allows fora large contact area between the die 206 and the package 204. A largercontact area 310 provides a stronger bond with the package 204 making itpossible to apply a larger pressure to the die 206.

Referring to FIG. 4 illustrates a perspective view of a pressure sensor200 depicting counterbore depth, which can be implemented in accordancewith a preferred embodiment. Note that in FIG. 3 and FIG. 4, identicalor similar parts or elements are indicated by identical referencenumerals. Thus, FIG. 4 also contains the counterbore 202 and backingplate 205

FIG. 5 illustrates a graph 500 depicting variation of TCO with respectto counterbore depth, which can be implemented in accordance with apreferred embodiment. As seen in the graph 500 the counterbore 210depicted in FIG. 2 can be modified to minimize the TCO. For example theTCO was decreased by factor of 4 by using the counterbore 202. The depthand diameter of the counterbore 202 can be adjusted to find the backingplate's geometry that minimizes the TCO.

FIG. 6 illustrates a high level flow chart 600 depicting the process ofreducing TCO with a counterbore 202, in accordance with a preferredembodiment. As depicted at block 601, the counterbore 202 can beemployed on backing plate 205 to improve stress isolation. Next asdescribed at block 602, the depth and diameter of counterbore 202 can beadjusted. Thereafter, as indicated at block 603, the geometry allowingbacking plate 205 to absorb more package stresses can be found. Next, asdepicted at block 604, the geometry can be employed to reducetemperature coefficient of offset on backing plate 205.

A backing plate with a counterbore can be used for stress isolation on apressure sensor, an accelerometer or any MEMS device that is sensitiveto package stresses.

It will be appreciated that variations of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims.

1. A sensor, comprising: a sensor die including a diaphragm and one ormore sensing elements coupled to the diaphragm; a sensor package forhousing the sensor die, wherein the sensor package includes a sensorport; and a backing plate for supporting the sensor die in the sensorpackage, wherein the backing plate includes an opening extendingtherethrough for exposing the diaphragm of the sensor die to the sensorport of the sensor package, wherein the opening defines a firstcross-sectional area adjacent to the sensor die, and has a transition toa second cross-sectional area adjacent to the sensor package, whereinthe second cross-sectional area is smaller than the firstcross-sectional area.
 2. The sensor of claim 1, wherein the backingplate includes a material having a coefficient of thermal expansion(CTE) that is substantially similar to that of the sensor die.
 3. Thesensor of claim 1, wherein the backing plate includes at least one ofsilicon, Pyrex, Hoya, and Borofloat.
 4. The sensor of claim 1, whereinthe transition of the opening between the first cross-sectional area andthe second cross-sectional area is a step transition.
 5. The sensor ofclaim 1, wherein the first cross-sectional area has a first diameter,and the second cross-sectional area has a second diameter, wherein thefirst diameter is larger than the second diameter.
 6. The sensor ofclaim 1, wherein the opening includes a bore that extends from a firstside of the backing plate through to a second side of the backing plate,with a counterbore extending from the first side of the backing platebut not all the way through to the second side of the backing plate,wherein at least part of the counterbore has the first cross-sectionalarea and at least part of the bore has the second cross-sectional area.7. The sensor of claim 6, wherein the first side of the backing platefaces the sensor die, and the second side of the backing plate faces thesensor package.
 8. The sensor of claim 7, wherein the first side of thebacking plate is secured to the sensor die, and the second side of thebacking plate is secured to the sensor package.
 9. The sensor of claim6, wherein the second cross-sectional area of the opening provides arelatively larger contact area between the backing plate and the sensorpackage than the first cross-sectional area provides between the backingplate and the sensor die.
 10. The sensor of claim 1, wherein the sensordie includes silicon.
 11. The sensor of claim 10, wherein the backingplate includes at least one of silicon, Pyrex, Hoya, and Borofloat. 12.A sensor, comprising: a sensor die including a diaphragm and one or moresensing elements coupled to the diaphragm; a sensor package for housingthe sensor die; a backing plate situated between the sensor die and thesensor package, a first side of the backing plate facing the sensor dieand a second side of the backing plate facing the sensor package; thebacking plate including a cavity; and the backing plate defines a wallbetween at least part of the cavity and the second side of the backingplate, making the backing plate less rigid and allowing the backingplate to absorb more stress.
 13. The sensor of claim 12, wherein thecavity includes a counterbore extending laterally out from a bore. 14.The sensor of claim 12, wherein the second side of the backing plate issecured to the sensor package.
 15. The sensor of claim 14, wherein thefirst side of the backing plate is secured to the sensor die.
 16. Thesensor of claim 12, wherein the cavity extends from the first side ofthe backing plate but not all the way through to the second side of thebacking plate.
 17. The sensor of claim 12, wherein the backing plateincludes a material having a coefficient of thermal expansion (CTE) thatis substantially similar to that of the sensor die.
 18. A method forimproving package stress isolation in a sensor package assembly,comprising: providing a bore through a backing plate; providing acounterbore along only part of the bore of the backing plate to improvestress isolation of the backing plate, wherein the counterbore enters afirst side of the backing plate and decreases a stiffness of the backingplate; mounting a sensor die adjacent to the first side of the backingplate and adjacent the counterbore, thereby forming a backingplate/sensor die assembly, wherein the sensor die includes a diaphragmthat in fluid communication with the counterbore and bore of the backingplate; and mounting the backing plate/sensor die assembly in a sensorpackage to form the sensor package assembly, wherein the sensor packageincludes a sensor port that is in fluid communication with the diaphragmvia the counterbore and bore of the backing plate.
 19. The method ofclaim 18, wherein the counterbore has a first cross-sectional area andthe bore has a second cross-sectional area, wherein the secondcross-sectional area is smaller than the first cross-sectional area. 20.The method of claim 19, wherein the first cross-sectional area has afirst diameter, and the second cross-sectional area has a seconddiameter, wherein the first diameter is larger than the second diameter.